It is barely worth pointing out that Donald Trump's surprise visit to Mexico on Wednesday won't do President Enrique Peña Nieto much good. Peña Nieto is deeply unpopular in his home country, with a quarterly survey from the newspaper Reforma putting his favorability at 23 percent — a figure so low that it makes Trump himself, at 35 percent, seems positively embraced.

That 35 percent is in the United States, of course. In Mexico, Trump's a lot less popular. A June survey showed Trump at 75 percent unfavorability in the country — compared with Hillary Clinton's 6 percent. When Ipsos asked people around the world in June who they'd pick in the American presidential...